Fracture studies in superalloys

Thesis



This research is concerned with the effects of temperature and frequency on the fatigue crack propagation behaviour. The fatigue behaviour at elevated temperature has been reviewed with emphasis on the mechanisms of propagation proposed to explain the effects of temperature and frequency.

The crack propagation characteristics of a single crystal superalloy have been investigated at various temperatures and frequencies. Determination of the fatigue crack propagation rates was combined with the measurement of crack tip plastic zone sizes and observation of the deformation behaviour.

At elevated temperature, the fatigue crack propagation rates were determined to be dependent on the frequency of loading. At low and high crack growth rates increased crack propagation rates were associated with higher cycling frequencies. At intermediate crack growth rates, the highest propagation rates were associated with the lowest frequency.

Selected area channelling pattern techniques were used to determine the plastic zone sizes and revealed that at high frequencies (>10Hz) the plastic zone sizes at elevated and ambient temperature are similar. With decreasing frequency at elevated temperature creep may lead to increased plastic zone sizes but may also affect the crack tip geometry thereby restricting further effects.

At room temperature crack propagation is observed to occur by shear of the γ' precipitate on <110> {111} resulting in {111} crystallographic facets on the fracture surface. At elevated temperature (>6000 C) however, the crack growth mechanism changes and propagation takes place on {100}. Observations of the dislocation structure indicate that deformation is concentrated within the γ matrix. It is proposed that the observed crack propagation in the γ matrix on {100} arises by the constraints imposed by the surrounding γ' precipitate. A model is presented for the frequency dependence of the intermediate crack growth rate in terms of the observed strain gradients within the plastic zone and the achievement of a critical local fracture strain.

Authors: Crompton, J; Crompton, J. S.

• Publication date: 1983
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Effect of temperature on; Alloys; Fatigue